Connection lead for an electrical accessory device of an mri system

ABSTRACT

A magnetic resonance imaging apparatus is provided with one or more electrical accessory devices, for example, catheters ( 10 ) or RF body coils ( 6 ), which are intended for use during the examination of an object, as well as with a connection lead ( 13 ) which is arranged so as to extend through an examination zone ( 1 ) of the magnetic resonance imaging apparatus, which zone can be exposed to an RF field, and to connect the accessory device to a connection unit ( 12 ). In order to avoid heating of the connection lead ( 13 ) due to currents induced in the connection lead by the RF field, which currents could lead to injury of a patient or damage of the accessory device or the connection unit ( 12 ), the connection lead ( 13 ) comprises at least one lead segment ( 131, 132 , . . . ) which has a length which is limited by at least one inductive coupling element, e.g. a transformer ( 141, 142, . . . ; 161, 162 , . . . ) and is unequal to n*/2, where denotes the RF wavelength and n=1, 2, 3, . . . .

The invention relates to a magnetic resonance imaging apparatus which isprovided with one or more electrical accessory devices such as, forexample, RF body coils or catheters with coil elements which areintended for use during the examination of a patient or other object, aswell as with a connection lead which is to be guided through anexamination zone of the magnetic resonance imaging apparatus, which zonecan be exposed to an RF field, and which lead is intended to connect theaccessory device to a connection unit such as, for example, a powersupply or control unit.

A magnetic resonance (MR) imaging apparatus is used in particular forthe examination and treatment of patients. The nuclear spins of theobject to be examined are then aligned by a steady main magnetic field(B₀ field) and are excited by RF pulses (B₁ field). The relaxationsignals thus formed are exposed to gradient magnetic fields for thepurpose of localization and are received in order to form in knownmanner therefrom an image of the tissue examined.

Essentially two types of construction can be distinguished, that is, theso-called open MR systems (vertical systems) in which a patient isintroduced into an examination zone which is situated between the endsof a C-arm and hence is accessible from practically all sides, that is,also during the examination or treatment, and also MR systems whichcomprise a tubular examination space (axial systems) in which thepatient is introduced.

RF coil systems serve for the transmission of the RF signals and thereception of the relaxation signals. In addition to the RE coil systemswhich are permanently built into the MR imaging apparatus, use is alsomade of RF body coils which can be flexibly arranged, for example, as asleeve or pad around or on the region to be examined.

Furthermore, use is made of catheters which are introduced into thepatient, for example, in order to take a sample of tissue during theimaging and which comprise a coil element, an oscillator or the like atthe area of their tip for the purpose of localization in the imageformed.

Accessory devices of this kind and other kinds are to be connected, viaan electrical connection lead, to a connection unit, notably a powersupply, a receiving device and/or a control device, which is situatedoutside the examination zone.

A problem in this respect is posed by the fact that the electrical fieldgenerated by the RF coil systems induces RF currents in the electricalconnection lead leading to the relevant accessory device; these currentsinvolve not only the risk of disturbances or destruction of theaccessory device and the connection unit, but notably can give rise tosubstantial heating of the connection lead and, in the case of bodycoils and catheters, to burning of the patient when the leads are tooclose to the patient.

U.S. Pat. No. 6,284,971 discloses various coaxial cables for use inmagnetic resonance imaging where the risk of burning of a user is to beavoided by a different configuration of the outer insulation of thecable. This outer insulation consists of a cylindrical inner shieldingportion which encloses the conductor as well as of a segmented outershielding portion, which portions are connected to one another. Betweenthese shielding portions there may be situated a dielectric materialhaving a comparatively high relative permitivity. In other embodimentsconductive elements are provided at the ends of the segmented outershielding portions, or such ends are connected to the inner shieldingportion via a capacitor.

Cable structures of this kind, however, are comparatively voluminous,complex and expensive and the results that can be achieved thereby inrespect of suppression of signals induced by the RF pulses are ofteninadequate, in particular in the case of high RF field strengths.

Therefore, it is a general object of the invention to provide apossibility of avoiding at least substantially the risk to a patientwhich is caused by the heating of leads guided through an examinationzone of a magnetic resonance imaging apparatus.

It is notably an object to provide a magnetic resonance imagingapparatus with one or more accessory devices, such as RF body coils andcatheters, in which the currents induced by RF pulses (B₁ field) in theconnection leads leading to these accessory devices do not constitute arisk for the patient or the accessory device or the connection unit.

It is also an object to provide an accessory device of the kind setforth with an electrical connection lead which enables an at leastsubstantially disturbance-free connection to be established with aconnection unit, for example, a power supply device, receiving deviceand/or control device, during use in an examination zone of an MRimaging apparatus, that is, without the risk of burning of a patient bythe connection lead or of damaging of the connection unit by RF currentsinduced in the connection lead.

The object is achieved in conformity with claim 1 by means of a magneticresonance imaging apparatus which is provided with at least oneelectrical accessory device for use during the examination of an object,as well as with a connection lead which is to be guided through anexamination zone of the magnetic resonance imaging apparatus, which zonecan be exposed to an RF field, and which lead is intended to connect theaccessory device to a connection unit, at least one lead segment, havinga length which is limited by at least one inductive coupling element andis unequal to n*λ/2, being connected in the connection lead, where λdenotes the RF wavelength and n=1, 2, 3, . . . .

The object is also achieved by means of an RF body coil acting as anaccessory device in conformity with claim 10, and by means of a catheterwith a transmission and/or receiving unit acting as an accessory devicein conformity with claim 11.

Special advantages of these solutions consist inter alia in that theendangering of the patient by heating of the connection lead is reliablyprecluded for practically all field strengths of the RF field so thatthe connection lead can be installed directly in the bed of the patient.The risk of damaging of a connection unit connected to the connectionlead, notably by RF currents induced in the connection lead, is at leastsubstantially precluded. Furthermore, in comparison with othersolutions, for example, an optical transmission link with opticalfibers, significantly fewer modifications of the components to beconnected are required. Finally, the connection lead in accordance withthe invention can also be realized so as to have a very smallcross-section (for example, less than 2 mm); this is of importance inparticular with a view to the application involving catheters.

The dependent claims relate to advantageous further embodiments of theinvention.

The development of heat around the conductor segment is particularlyeffectively suppressed by means of the embodiment disclosed in claim 2.

The claims 3 to 5 relate to preferred embodiments of the inductivecoupling element, whereas claim 6 discloses preferred embodiments of theconnection lead.

The embodiment in conformity with claim 7 is particularly advantageouswhenever the connection lead also has to conduct a direct voltage.

The embodiment disclosed in claim 8 further enhances the transmission ofthe signals to be evaluated, for example, the MR relaxation signalspicked up by a body coil.

Finally, claim 9 discloses two preferred accessory devices.

Further details, features and advantages of the invention will becomeapparent from the following description of preferred embodiments whichis given with reference to the drawing. Therein:

FIG. 1 is a diagrammatic side elevation of an MR imaging apparatus;

FIG. 2 is a diagrammatic representation of an accessory device;

FIG. 3 shows a first equivalent diagram of a connection lead inaccordance with the invention;

FIG. 4 is a diagrammatic representation of a first embodiment of theconnection lead;

FIG. 5 is a diagrammatic representation of a second embodiment of theconnection lead;

FIG. 6 shows a transformer used in the connection lead in conformitywith the FIGS. 4 and 5;

FIG. 7 is a diagrammatic representation of a third embodiment of theconnection lead, and

FIG. 8 shows a second equivalent diagram of a connection lead inaccordance with the invention.

FIG. 1 shows the components of an open MR imaging apparatus which are ofessential importance in relation to the generation and picking up ofmagnetic fields in an examination zone 1. Above and underneath theexamination zone 1 there are provided respective magnet systems 2, 3which serve to generate an essentially uniform main magnetic field (B₀field for magnetizing the object to be examined, that is, for aligningthe nuclear spins) whose magnetic flux density (magnetic induction) maybe of the order of magnitude of from some tenths of Tesla to some Tesla.The main magnetic field essentially extends through a patient P in adirection perpendicular to the longitudinal axis of the patient (thatis, in the x direction).

Planar or at least approximately planar RF conductor structures (surfaceresonators) in the form of RF transmission coils 4 serve to generate RFpulses (B₁ field) of the MR frequency whereby the nuclear spins areexcited in the tissue to be examined, said RF transmission coils beingarranged on the respective magnet systems 2 and 3. RF receiving coils 5serve to pick up subsequent relaxation events in the tissue; these coilsmay also be formed by surface resonators provided on one of the magnetsystems 2, 3. A common RF surface resonator can also be used fortransmission and reception if it is suitably switched over, or the twoRF surface resonators 4, 5 can serve for the alternating transmissionand reception in common.

Furthermore, for the spatial discrimination and resolution of therelaxation signals emanating from the tissue of a patient P(localization of the excited states) there are also provided a pluralityof gradient magnetic field coils 7, 8 whereby three gradient magneticfields are generated which extend in the direction of the x axis. Afirst gradient magnetic field then varies essentially linearly in thedirection of the x axis, while a second gradient magnetic field variesessentially linearly in the direction of the y axis, and a thirdgradient magnetic field varies essentially linearly in the direction ofthe z axis.

Electrical accessory devices are required for given examinations. Suchdevices are, for example, RF body coils 6 which are used in addition toor as an alternative for the planar RF receiving coils 5 and which arearranged as RF receiving coils directly on the patient P or the zone tobe examined. These RF body coils 6 are generally constructed as flexiblepads or sleeves.

Furthermore, in order to carry out the treatment on the patient P or toextract a tissue sample or to determine tissue parameters, use is oftenmade of a catheter 10 which is introduced into the patient and whoseposition is to be visualized on a display screen.

Various passive and active methods are known for this purpose.

In the case of a passive method, for example as described in WO99/19739, one or more small resonant oscillatory circuits on the tip ofthe catheter can be made visible in the MR image because of the factthat they lead to an increase of the RF field (B₁ field) in their directvicinity during MR imaging, and hence also increase the magnetization ofthe neighboring nuclear spins. The transmission and/or receiving unit 11is then formed by a receiving coil in the simplest case. However, it mayadditionally comprise sensors which pick up given properties of thesurrounding tissue.

In the case of an active method it is possible to switch between twomodes of operation in an alternating fashion, for example, by means of aswitching unit 41 which is connected to the catheter 10 by way of afirst output A and to the RF transmission coils 4 by means of a secondoutput B. In the first mode of operation an MR image is generated inknown manner by means of the MR apparatus, whereas in the second mode ofoperation a local nuclear magnetization is excited, using an activatedtransmission and/or receiving unit 11 which is arranged on the tip ofthe catheter, by transmission of RF pulses, the resultant relaxationevents being received by the RF receiving coils 5, 6. The signalreceived itself serves to reproduce the position of the tip of thecatheter in the MR image.

FIG. 2 is a diagrammatic representation of an accessory device in theform of a catheter. On the tip of the catheter (or in a location at aslight distance therefrom) there is arranged a transmission and/orreceiving unit 11, for example, in the form of a microchip on which thenecessary components (and possibly also the sensors) are realized. Atthe end of the catheter which is situated outside the patient there isprovided a connection unit 12 in the form of a power supply unit and/ora receiving device and/or a control device which is connected, via aconnection lead 13 which is guided through the catheter, to thetransmission and/or receiving unit 11 and via which the transmissionand/or receiving unit 11 is activated and possibly the measuring valuesfrom the sensors are conducted.

In the case of an accessory device in the form of RF body coils 6, suchcoils are also connected, via an electrical connection lead 13, to acorresponding connection unit 12 (power supply, receiving device and/orcontrol device).

FIG. 3 shows a first electrical equivalent diagram of a connection lead13 in accordance with the invention; the operating principle of theembodiments shown in the FIGS. 4, 5 and 7 will be illustrated on thebasis thereof.

The RF pulses (B₁ field) transmitted by the RF transmission coils 4induce, for example, in an RF body coil 6 as well as in the part of theconnection lead 13 which extends through the field of the RFtransmission coils 4, a common mode signal which is generated by a firstvoltage source U₁ in the equivalent diagram. The common mode signalcauses a corresponding first current I₁ in the connection lead 13. Thesignals induced by the subsequent MR relaxation events in the RF bodycoil 6 (differential mode signals) are represented by a second voltagesource U₂ (useful voltage) in the equivalent diagram and give rise to asecond current I₂ in the connection lead 13.

The connection lead 13 has a plurality of lead segments 131, 312, . . .. The length of these segments is unequal to n*λ/2 (n=1, 2, 3, . . . ),where λ is the wavelength with which the RF pulses are transmitted. Thesegments 131, 132, . . . are, therefore, non-resonant for the commonmode signal. The length of the segments is preferably as small aspossible and lies notably between λ/4 and λ/8. Respective transformers141, 142, . . . , are provided for connecting the individual leadsegments 131, 132, . . . to one another; the differential mode signalscan be transmitted via said transformers so as to be conducted via theconnection lead 13. The transformers 141, 142 are proportioned such thatthe coupling capacitance C between the primary side and the secondaryside is as small as possible and preferably not smaller than 250 Ohm (orlarger than 250 Ohm in an absolute sense).

A significant temperature increase at the area of the patient is thusavoided even in the case of high RF field strengths (for example, 3Tesla) as well as in the case of a large number of RF coils 4, thusavoiding damaging and/or failure of the accessory device 6 and theconnection unit 12.

In the case where the RF body coil is composed of a plurality ofindividual conductor segments (antenna segments) which can be connectedto one another or separated from one another by means of diodes in orderto achieve given reception characteristics, the power supply and theswitching of the diodes can be realized by means of alternating voltagesignals which are generated by the connection unit 12 and conducted viathe connection lead 13. At a frequency of, for example, 2 MHz of thepower supply and of, for example, 20 MHz of the switching voltage (thatis, frequencies clearly beyond the range of the MR frequency, but withinthe transmission bandwidth of the connection lead), the connection lead13 exhibits no significant attenuation in this respect.

The connection lead 13 can be realized, for example, in conformity witha first embodiment as shown in FIG. 4. This is a two-wire lead (forexample, a twisted pair), three lead segments 131, 132, 133 of which areshown. The lead segments are coupled to one another via a respectivetransformer 141, 1412 whose primary and secondary windings L1, L2terminate the respective lead segment. Optionally, the lead segments131, 132, 133 may be provided with a shield 151, 152, 153; the shieldsthen overlap one another in a contactless manner at the area of thetransformers 141, 1412.

FIG. 5 is a diagrammatic representation of a second embodiment of theinvention in which a coaxial cable is used as the connection lead 13instead of the two-wire lead shown in FIG. 4. In this case the primaryand secondary windings L1, L2 of the transformers 141, . . . areconnected between the conductor Lt and the shielding A of the individualsegments of the coaxial cable.

In conformity with FIG. 6 the transformers 141, 142 may comprise, forexample, a toroid T on which the primary winding L1 and the secondarywinding L2 are wound. The two windings L1, L2 may also encompass theentire toroid T and be arranged one over the other.

The material of the toroid T should have an as low as possible relativepermitivity and the winding wires should be as thin as possible. Anattenuation of less than 1 dB can thus be achieved. In the case oftransformers which are situated outside the range of the main magneticfield, the toroid may also be made of a magnetic material wherebyparticularly favorable properties can be achieved.

Alternatively, if desired, a metallic transformer core can also bedispensed with and the transformer may be composed of air coils woundaround a coil former made of a foamy material.

At both ends of the connection lead 13 the transformers may beconstructed so as to form part of the RF body coil 6 (or a transmissionand/or receiving unit 11 of a catheter 10) or of a connector on theconnection unit 12.

When the (discrete) transformers 141, 142, . . . are not desired alongthe connection lead 13 for mechanical or other reasons, it is possibleto realize the transformers in the form of conductor loops 161, 162, . .. . FIG. 7 shows such a third embodiment of the connection lead 13; thisembodiment is advantageous notably when the connection lead 13 must havea particularly small cross-section.

This connection lead 13 is again composed of a plurality of leadsegments 131, 132, 133 with two cores, which are short-circuited at therespective ends of each lead segment. The conductor segments are againinductively coupled to one another. To this end use is made of saidconductor loops 161, 162 which are arranged each time over end zones ofneighboring lead segments 131, 132 and 132, 133 etc. This connectionlead 13 can be realized, for example, by way of a strip-like board orother, also flexible carrier material (for example, a foil) which isprovided on one side with the lead segments 131, 132, 133, . . . andwith the conductor loops 161, 162, . . . on the other side.

Optionally, shields 171, 172; 173, 174 may also be provided in thisthird embodiment, said shields being arranged on the conductor loops161, 162 and/or the lead segments 131, 132, 133.

Finally, FIG. 8 shows a second equivalent diagram illustrating a fourthembodiment of a connection lead in accordance with the invention.

In this equivalent diagram the voltage generated by a first voltagesource U₁ again represents the voltage which is induced, by the RFpulses emitted by the RF transmission coils 4, in an RF body coil 6 aswell as in the part of the connection lead 13 which extends through thefield of the RF transmission coil 4 (common mode signal). A secondvoltage source U₂ represents the (differential mode) signals induced inthe RF body coil 6 by the MR relaxation events. The two lead segments131, 132 shown in FIG. 8 are again connected to one another via atransformer having a primary winding L1 and a secondary winding L2 inconformity with the foregoing description. The transformer is shown inthe form of a known T equivalent circuit consisting of a parallel mutualinductance M of the two windings L1, L2 as well as the serialinductances L1-M and L2-M.

Essential in this respect is a first capacitor C1 which is connected inseries with the first lead segment 131, as well as a second capacitor C2which is connected in series in the second lead segment 132. Thecapacitance of the capacitors is chosen to be such that they form aresonant circuit in conjunction with the inductance of the transformer,that is, 1/ωC1=ωL1 and 1/ωC2=ωL2, and that this resonance condition issatisfied for the circuit frequency ω of a signal to be conducted viathe connection lead, that is, for the differential mode signal, but notfor the common mode signals.

A very high and at the same time very narrowband coupling of the leadsegments 131, 132 can thus be achieved for the MR relaxation signals.Moreover, the coupling capacitance C between the windings L1, L2 of thetransformer can thus be kept even smaller.

In as far as direct voltage signals are to be conducted via theconnection lead 13, for example, in order to bias diodes between partsof the body coil 6, the two capacitors C1, C2 as well as theintermediate transformer can be bridged by means of ohmic resistances R.In respect of the bridging of the transformer, of course, this alsoholds in this sense for the first equivalent diagram shown in FIG. 3(not depicted therein).

The described connection leads offer special advantages for theapplication of switchable RF body coils 6 which are used notably in thecase of SENSE (Sensitivity Encoding) imaging methods, because on the onehand disturbance-free power supply and switching over of the variousparts of the RF body coils 6 by means of diodes as well as the transferof the received relaxation signals is thus possible as described above,while on the other hand there is no risk of the patient being burnt dueto resonance effects caused by the RF power emitted by the RFtransmission coil 4 and the inherent heating of the connection lead 13.The connection lead 13 can thus be arranged directly in the bed of thepatient. The risk for the accessory device 6, 11 or the connection unit12 is also precluded to a high degree. The same also holds for high RFfield strengths.

The use of such connection leads requires substantially fewer systemmodifications than, for example, the optical transmission of therelevant signals from and to the RF body coils, catheters or otheraccessory devices.

In comparison with the known solutions, notably the connection leads 13in conformity with the first up to and including the third embodimenthave a comparatively large bandwidth so that, for example, it is alsopossible to transfer a plurality of receiving signals via a connectionlead.

Finally, the same or even simpler connectors can be used for connectingthe connection lead 13 to the relevant connection unit 12 (power supply,receiving device and/or control device).

1. Method for the production of cellulose carbamate moulded bodiescomprising the following steps: a) producing a mixture of activatedcellulose or activated pulp and urea in a mixing system with shearingtreatment; b) drying of the mixture; c) causing reactive conversion attemperatures between 120° and 180° C.; d) dissolving the untreatedreaction product in diluted alkali lye; e) filtering and/or de-aeratingthe alkaline solution ; and f) shaping the cellulose carbonate into themoulded article. 2-18. (canceled)
 19. The method of claim 1 wherein thecellulose or pulp is activated with approximately 3 to 25% alkali lye.20. The method of claim 1 wherein the cellulose or pulp is activatedwith approximately 4 to 20% alkali lye.
 21. The method of claim 19wherein the residual alkali content of the cellulose or the pulp isreduced by washing and by subsequent pressing out or centrifuging toapproximately 0.1 to 0.5 mol alkali hydroxide per mol anhydroglycose.22. The method of claim 19 wherein the residual alkali content of thecellulose or the pulp is reduced by washing and by subsequent pressingout or centrifuging to approximately 0.15 to 0.3 mol alkali hydroxideper mol anhydroglycose.
 23. The method of claim 21 wherein the ratio ofurea to anhydroglycose unit is between approximately 0.5 and 5 relativeto the molecular mass.
 24. The method of claim 21 wherein the ratio ofurea to anhydroglycose unit is between approximately 0.8 and 3 relativeto the molecular mass.
 25. The method of claim 1 wherein the step ofdrying the reaction mixture is effected by temperature and vacuumtreatment.
 26. The method of claim 1 wherein the step of causingreactive conversion is effected at temperatures between approximately125 and 150° C.
 27. The method of claim 1 wherein the step of causingreactive conversion takes between approximately 30 and 240 minutes. 28.The method of claim 1 wherein the step of causing reactive conversiontakes between approximately 60 and 120 minutes.
 29. The method of claim1 wherein the step of causing reactive conversion is effected underinert or vacuum treatment.
 30. The method of claim 1 wherein a kneaderor an intensive mixer with forced conveyance is used as the mixingsystem.
 31. The method of claim 30 wherein the steps of producing of themixture, drying of the mixture and causing reactive conversion areimplemented in the same mixing system.
 32. The method of claim 30wherein the steps of producing of the mixture and drying of the mixtureare implemented in the intensive mixer and the step of causing reactiveconversion is implemented in the kneader.
 33. The method of claim 1further comprising the step of cooling the reaction product to belowapproximately 20° C. before dissolving.
 34. The method of claim 1further comprising the step of cooling the reaction product to betweenapproximately −10 and 10° C. before dissolving.
 35. The method of claim1 wherein the step of dissolving is effected using an approximately 3 to10% alkali lye.
 36. The method of claim 1 wherein the step of dissolvingis effected using an approximately 5 to 8% alkali lye.
 37. The method ofclaim 1 wherein the step of dissolving further comprises addingreinforcing fibers or expanding agents.
 38. The method of claim 1wherein the step of shaping is effected in a regenerating bath.
 39. Themethod of claim 1 wherein the step of shaping is effected by thermalcoagulation.
 40. The method of claim 1 wherein further comprising thestep of converting the molded article into regenerated material in analkaline manner.
 41. The method of claim 1 wherein the step of shapingresults in a molded article selected from the group of fibers, films,beads, sponges and sponge-type cloths.